What is "On Demand Scheduler"?

Is a collection of self 'schedulable' tasks that are given a Spring's TaskScheduler will get scheduled on demand / programmatically.

Why would I need it?##

Good question. Afterall, there is Spring's "<task:scheduled>", Quartz, JDK Timer, you name it...

The idea behind "On Demand" scheduler is to achieve three things:

1. Be very simple
2. Be based on Spring
3. Be able to be schedule tasks on demand
   

Currently, the way to schedule a task in Spring is to use <task:scheduled> ( satisfies #1 and #2 ):

<task:scheduled-tasks scheduler="myScheduler">
    <task:scheduled ref="someObject" method="someMethod" fixed-delay="5000"/>
</task:scheduled-tasks>

<task:scheduler id="myScheduler" pool-size="10"/>

Which means that the task is going to be scheduled automatically when the application context is created.

"On Demand" scheduler compliments this ability with 'Schedulable' tasks that can be scheduled exactly when needed.

Can you give me an example?

Business requirement: Whenever certain business service is done, schedule a pickup task ( e.g. to start picking up files every 5 seconds ).

<bean id="filePickupTask" 
      class="org.gitpod.scheduler.task.factory.FixDelayTaskFactoryBean">
    
    <property name="delay" value="5000"/>
    <property name="targetObject" ref="fileGrabber"/>
    <property name="targetMethod" value="grab"/>
    <property name="scheduler" ref="taskScheduler"/>
</bean>

That is pretty much it.

This will create an immutable task that can be injected anywhere and can be scheduled as:

deliveryTask.schedule();

To fully satisfy the above business requirement, you would of course have an after / around advice applied to that "certain method" that needs attention ( e.g. businessService() ), and have this task injected into the aspect.

---

Another example: Start process all the shipments of Ubuntu DVDs to Apple Headquarters every 5 minutes while the store is closed e.g. from 21:00 to 08:00

<bean id="shipOrder"
      class="org.gitpod.scheduler.task.factory.RunnableTaskFactoryBean">

    <property name="targetObject" ref="shipper"/>
    <property name="targetMethod" value="ship"/>
    <property name="arguments">
        <util:list>
            <value>Ubuntu DVD</value>
            <value>
                Apple Computer Inc 
                1 Infinite Loop
                Cupertino, CA 95014</value>
        </util:list>
    </property>
</bean>

Again, this is a simple Spring bean that can be injected anywhere, and the Spring's TaskScheduler APIs can be used. e.g. here is an example from "SchedulingRunnableTaskWithTriggerIntegrationTest":

@Before
public void shouldCreateTriggerAndScheduleTask() {

    long now = Calendar.getInstance().getTime().getTime();

    // Set a trigger to start in 5 second, run every 2 seconds for 15 seconds:
    Date startTime =  new Date( now + 5000 );
    Date endTime =  new Date( now + 15000 );
    long period = 2000;

    Trigger trigger = new DurationTrigger( startTime, endTime, period );

    taskScheduler.schedule( shipOrderTask, trigger );
}

where a "taskScheduler" is a regular Spring's "task:scheduler" and "org.gitpod.scheduler.trigger.DurationTrigger" is a custom trigger. You can simply grab it form sources.

The gist of it

The main idea here is to have these simple tasks: FixDelayTask, TimeOutTask, RunnableTask, FixRateTask, CronTask, etc., some of which are suggested by Spring APIs ( e.g. take a look at Spring's ScheduledTaskRegistrar ), inject them in to components that need them as 'Schedulable's, and schedule whenever appropriate.

TODO: more tasks, more tests, more love